Silvery-tone matte-finish flash coat alloy product

ABSTRACT

Matte-finish tinplated flat rolled steel product having about 0.01 to 0.03 pound per base box inner coating of tin-iron alloy and an outer coating of matte-finish tin sufficient to bring the total coating weight to about 0.1 pound per base box, produced by applying a flash coating of tin to the steel product, alloying the flash coating of tin with the base metal, and electroplating a matte-finish layer of free tin to the alloy coated product.

United States Patent Lamantia [151 3,652,234 [451 Mar. 28, 1972 [54] SILVERY-TONE MATTE-FINISH FLASH COAT ALLOY PRODUCT [72] inventor: Anthony J. Lamantia, Toronto, Ohio [73] Assignee: National Steel Corporation 221 Filed: May 20, 1969 [21] Appl. No.: 842,059

Related US. Application Data [62] Division of Ser. No. 624,699, Mar. 21, 1967, abandoned.

[52] 0.8. CI ..29/l83.5, 29/ 196.4 [51] Int. Cl. B210 27/00 [58] Field of Search ..29/ 196.4, 183.5; 204/37 T [56] References Cited UNITED STATES PATENTS 3,174,917 3/1965 Lesney et al ..29/ 196.4 X 3,285,838 11/1966 Morgan et al. 3,481,841 12/1969 Ham et al ..29/i96.4 X

Primary Examiner-Allen B. Curtis Attorney-Shanley and O'Neil [57] ABSTRACT and electroplating a matte-fmish layer of free tin to the alloy coated product.

3 Claims, No Drawings SiLVERY-TONE MATTE-FINISH FLASH COAT ALLOY PRODUCT This application is a division of application, Ser. No. 624,699 filed Mar. 21, 1967, now abandoned.

This invention is concerned with a new tinplated flat rolled steel product and its manufacture.

Tinplated container stock available in the past has been Grade A tinplate has an inner coating of iron-tin alloy weighing about 0.05 pound per base box (217.78 sq. ft.) and an outer coating of bright-finish free tin weighing 0.25, and higher, pound per base box. Grade A tinplate has high corrosion resistance but, because of the number of process steps required and the coating weight of tin required, is costly to manufacture and use is ordinarily limited to high acid-content foods.

Conventional tinplate refers to tinplated steel having a tin coating weight of 0.25, and higher, pound per base box. Conventionally, and as referred to herein, tin coating weights are expressed in pounds per base box and represent the total weight of tin coating on both surfaces of a sheet or strip. The tin coating is applied by electroplating followed by a flow brightening process in which the tin coating is melted instantaneously and then quenched to produce a bright-finish product. Conventional tinplate has been a satisfactory product but, because of manufacturing costs, uncoated steel (blackplate) or so-called tin-free steel has been replacing conventional tinplate in a number of container applications.

Blackplate has a number of disadvantages, such as, tendency to corrode rapidly both before processing into containers and after such processing. Further, soldering techniques for side seam and end seals of cans cannot be used on steel plate without a tin coating so that established and dependable processes and processing lines for manufacture of tinplate containers cannot be used. Therefore the need for lower cost tinplated container stock, providing substantially the protection of conventional tinplate and permitting processing by established methods and on established lines, has been felt for some time.

Decreasing the coating weight of the tin is no solution to the need for a lower cost, satisfactory, tinplated product. in the first place, coating weights below the conventional 0.25 pound per base box do not provide satisfactory or acceptable corrosion protection and the soldering ability of such lower coating weights makes them practically unusable on established processing lines. in addition, unusual surface marking of the product, known as woodgrain, has been more prevalent with coating weights less than 0.25 pound per base box.

The present invention provides a new coated product and method of manufacture which enable greatly reduced tinplate coating weight, to less than half the conventional minimum of 0.25 pound per base box, while providing corrosion resistance comparable to, and in many instances better than, that provided by 0.25, and higher, pound per base box tinplate. Further, while reducing the cost of the tinplating, the invention provides unusual advantages in color-tone, surface finish, soiderability, and adaptability to further processing which make the product of the present invention preferable to prior art tinplated container stock for many uses. It is significant that these advantages and the reduction in coating weight are provided without increasing manufacturing costs over those required for conventional tinplate.

in practice of the present invention, standard steel base stock, blackpiate, is prepared for coating by conventional rolling, heat treating and cleaning methods, which form a necessary part of the process of preparing stock for coating but need not be described for an understanding of the invention. After such pre-plating preparations, a flash coating of tin between about 0.01 and 0.03 pound per base box is applied to the flat rolled steel base stock. This can be applied on a conventional electrotinplating line, after which electroplating solution is rinsed from the product. in accordance with the invention, this flash coating of tin is converted to iron-tin alloy. This step is accomplished by heating the flash coated strip to about 425 to 550 F. for a sufficient periodof time to substantially completely alloy the tinplating with the base metal. The heating can be carried out in conventional high-frequency induction heating apparatus, of the type used for flow brightening conventional tinplate.

After alloying, a light coating of free tin, (elemental tin, as referred to in the claims) sufficient to bring the total coating weight on the flat rolled steel base metal to about 01 pound per base box is applied in an electroplating bath. ln accordance with the teachings of the invention, this free-tin coating is not flow-brightened and the electroplated mattefinish is maintained.

The flash coat alloy and the matte-finish free tin, in combination, produce a highly desirable silvertone appearance. This is believed to stem from the silvery finish of the lightweight alloy coat and the translucent quality of the light matte-finish free tin. Such a silvery finish is not available with substantially heavier alloy coatings which produce a bluishgrey finish.

The silver tone finish does not change with subsequent enameling heat treatment, that is the so-called stoving operations used in applying lacquer or enamel to that surface of the strip used on interior portions of a container. Stoving operations are carried at a temperature around 410 F. The favorable performance of the present product during stoving operations stems from an important contribution made by the invention, that is stabilization of the alioy'layer coating thickness. This stabilization has been verified by test results and data from these tests is set forth later in the specification.

The unusual finish taught by the present invention has other advantages in providing an attractive background for subsequent printing or labeling. In addition the matte-finish is more receptive to printing inks used in lithographing, and the like, than bright finished tinplate.

As stated above, the corrosion resistance of the product of the present invention has been found to be comparable to or better than conventional 0.25 pound per base box tinplate. This has been verified by iron Pickup Tests. These are standard and accepted tests in the art which provide an indication of internal container corrosion, and therefore shelf life of a container. These tests measure the iron pickup in parts per million in a preselected solution over specified periods of time. The iron going into solution is a measure of exposure of the iron in the'base metal. Iron going into solution also has deleterious effects on the color and taste of the container contents.

A carbonated soft drink was used as the solution in the following lron Pickup Tests. These tests indicate that the continuous alloy and matte finish of the present invention form a better shield than 0.25 pound per base box conventional tinplate.

Solderability of tinplated steel is dependent on the tin available in the coating. After normal flow-brightening and enamel stoving operations conventional-type tinplate having a coating weight lighter than 0.25 pound per base box apparently does not have sufficient free tin available to permit satisfactory soldering. Flow-brightening of conventional tinplate was required in the past to obtain the desired bright finish and to generate the proper tin crystalline structure for soldering which was not available with a matte finish on conventional tinplate.

The matte-finish 0.10 pound product of the present invention can be soldered readily, indicating that alloying the flash coating of the present invention stabilizes the alloy layer and maintains a sufficient amount of tin during stoving operations to provide satisfactory solderability notwithstanding the matte-finish.

The recognized test for solderability in the art is the capillary rise test" in which the length of rise of solder between folds of a piece of sheet metal of prescribed size and orientation is measured. Ordinarily container manufacturers require that the rise of solder be no less than five-sixteenths of an inch at any location and some specifications call for a higher average rise. Conventional tinplate 0.25 pound per base box after stoving operations shows a rise of between five-sixteenths and nine-sixteenths inch. Product of the present invention after stoving operations provide a rise between five-sixteenths and eight-sixteenths of an inch. Conventional-type tinplate of 0.10 pound per base box tests between zero and about two-sixteenths of an inch rise.

It should be noted that manufacturing steps in the present invention are not increased over those required for conventional tinplate since the flash-coat alloying step replaces the flow-brightening step practiced with conventional tinplate. Also, that the more economical product of the present invention provides substantially the corrosion protection and solderability of conventional tinplate and in addition provides an attractive finish which is more suitable for lithographing, and the like, than bright-finish tin.

What is claimed is:

l. Matte-finish electrotinplated steep strip having a uniform subsurface tin-iron alloy layer of approximately 0.03 pound per base box and an outer silvery-tone matte-finish elementaltin layer of a weight sufficient to bring the total coating weight on the steel strip to approximately 0.] pound per base box.

2. Coated product comprising electrotinplated steel strip having a subsurface tin-iron alloy layer of approximately 0.03 pound per base box and an outer silvery-tone matte-finish elemental-tin layer, with the total coating weight on the steel strip being approximately 0.] pound per base box.

3. A new tinplated flat rolled steel product of improved corrosion-resistance having a silvery-tone matteinish outer surface comprising flat rolled steel base metal having an inner coating of iron-tin alloy of uniform coating weight between about 0.01 and about 0.03 pound per base box and having a uniform matte-finish elemental-tin outer coating of a coating weight sufficient to bring the combined iron-tin alloy and elemental-tin coating weight on the flat rolled steel base metal to about 0.] pound per base box. 

2. Coated product comprising electrotinplated steel strip having a subsurface tin-iron alloy layer of approximately 0.03 pound per base box and an outer silvery-tone matte-finish elemental-tin layer, with the total coating weight on the steel strip being approximately 0.1 pound per base box.
 3. A new tinplated flat rolled steel product of improved corrosion-resistance having a silvery-tone matte-finish outer surface comprising flat rolled steel base metal having an inner coating of iron-tin alloy of uniform coating weight between about 0.01 and about 0.03 pound per base box and having a uniform matte-finish elemental-tin outer coating of a coating weight sufficient to bring the combined iron-tin alloy and elemental-tin coating weight on the flat rolled steel base metal to about 0.1 pound per base boX. 